Influence of temperature and phase transitions on the Urbach's tails of absorption spectra for TlInS2 single crystals

Optical absorption spectra of TlInS2 single crystals were investigated in the temperature range of 10-340 K. The exponentially increasing absorption tails were explained as Urbach tails. TlInS2 single crystals exhibited five different convergent groups of lines ln alpha(E,T) having different convergence points of E-0 and alpha(0) between 120 and 340 K. Characteristic Urbach's parameters such as steepness parameter [sigma(T)], Urbach's energy [E-u(T)], and effective phonon energy (h nu(p)) were obtained as a function of temperature. sigma(T) and E-u(T) exhibited a number of jump-like trends at the identified temperature regions. Jump-like changes of sigma(T) and E-u(T) were explained to be due to the phase transitions (PTs) and contributions of electron-(exciton)-phonon interaction caused by dissociation. The temperature dependence of the band gap (BG) exhibited six clearly distinct regions and differs from the temperature dependent BG behavior of known semiconductors. Absorption edge and band gap anomalies in TlInS2 samples at PT were interpreted using the theory proposed by Zametin [Phys. Status Solidi B 124, 625 (1984)]. Measured BG jumping values, Delta E-g(alpha), are greater than those calculated from this theory in the 340-200 and 175-120 K regions, while one of them is smaller than that of the calculated in the 198-180 K region. These differences were explained as due to PTs and simultaneous influence of phonon coupling with electrons (excitons) induced by exciton dissociation, and the existence of the chaotic states caused by the coexistence of different phases which have opposite sign contributions below the ferroelectric phase transition, respectively. (C) 1998 American Institute of Physics. [S0021-8979(98)04619-2].